An ad hoc network comprises portable wireless devices (nodes) that do not require fixed infrastructure. Wireless devices may become part of the network when they are located within the range of another node in the network. Each node in the ad hoc network may serve as a client, host, or router. The nodes are mobile devices, such as laptop computers, PDAs, among other mobile communication devices.
Currently, a number of wireless technologies exist for supporting ad hoc networks, including those operating under the Bluetooth, Infrared Data Association (IrDA), HiperLAN, and 802.11 standards. Potential applications of ad hoc networks illustratively include battlefield networks, emergency networks in disaster areas, among other network environments not requiring a stationary communications device, such as a base station for establishing communications between mobile devices.
The mobile nodes rely on battery power to operate, and consequently have power limitations. In order to extend the battery life for such wireless devices, various sleeping protocols have been implemented to force the nodes to turn off their radios when extended periods of communication inactivity are detected.
A node (host) in the active mode is fully powered, and thus may transmit and receive at any time. By contrast, a host operating in a power saving (sleep) mode, illustratively under the 802.11 standard, only wakes up periodically to check for possible incoming packets from other nodes. The nodes are grouped into clusters, where one of the nodes is selected as a cluster-head node, as conventionally known in the art. Periodically, the cluster-head transmits beacon frames spaced by a fixed beacon interval. Each beacon frame, delivers a traffic indication map (TIM), which contains ID's of those nodes with buffered unicast packets stored in the cluster-head. A node hearing its ID will stay awake for the remaining beacon interval.
Various sleeping protocols have been proposed to coordinate the sleep time of nodes. For example, one approach has been to impose a power saving mode to stay awake at for at least half of a beacon interval in each beacon interval. Accordingly, each node is awake long enough to ensure that neighboring nodes are aware of each others presence, such that buffered packets may be delivered, as required. However, since the clocks between the various nodes are not synchronized, this sleeping protocol requires that each host must stay in the active state more than half of the time, which is not very efficient for saving power.
Another proposed sleeping protocol is a “periodically-fully-awake-interval protocol. In this protocol, two types of beacon intervals are provided. The first interval is a low power interval protocol, where the length of the active window is reduced to the minimum, and the second interval is a fully-awake interval protocol, where the length of the active window is extended to the maximum. Since fully-awake intervals require a lot of power, they only appear periodically and are interleaved by low-power intervals. Although the energy required may be significantly reduced by using either of these two sleeping protocols, a node must still contend with other nodes to send a beacon in each beacon interval. Therefore, there is a need in the art for an improved sleeping protocol for ad hoc networks.